Flat light sources are virtual necessities in many technical fields, especially in the information display field. Typically, a flat light source having a uniform brightness is a vital component in passive displays such as liquid crystal displays. Conventionally, uniform flat lighting is generally obtained by optical manipulation techniques. For example, a backlight module of a typical liquid crystal display employs an optical system including several optical parts including a light guide plate. The optical system transforms a linear light source or a point light source into a flat light source.
Referring to FIG. 7, a conventional backlight module 10 for use in a liquid crystal display includes a light emitting diode (LED) 12, a light guide plate (LGP) 14, and a micro-lens 16 arranged therebetween. Divergent light beams emitted from the LED 12 are collimated into parallel light beams by the micro-lens 16, and the parallel light beams then propagate into the LGP 14. Subsequently, the light beams are uniformly output from a flat emitting surface of the LGP 14.
However, the above-described backlight modules cannot directly provide a planar light source. Intermediate optical manipulation is required, and some loss of light energy is inevitable. Furthermore, the optical parts such as the micro-lens 16 and the LGP 14 must be precisely manufactured and assembled. This increases manufacturing costs.
Field emission devices are based on emission of electrons in a vacuum in order to produce visible light. Electrons are emitted from micron-sized tips in a strong electric field, and the electrons are accelerated and collide with a fluorescent material. The fluorescent material then emits visible light. Field emission devices are thin and light, and provide high brightness. Up to the present time, light sources including field emission cathodes have been devised. One example is the field emission bulb. Nevertheless, there is no known device based on field emission principles which provides a satisfactory planar light source.